1. Field of the Invention
This invention relates to sub-surface borehole underreamers. More particularly, the invention relates to underreamers having large revolving diamond cutter elements that precess as the underreamer body rotates in a borehole.
2. Description of the Prior Art
A sub-surface borehole underreamer is a tool which is used to enlarge a portion of the length of a hole drilled in the earth below a restriction in the hole. Such tools are used in drilling oil, gas, water, mining, and construction holes in wells, and also in the formation of shot-holes for blasting. An underreamer has two operative states, a collapsed or closed state in which the tool diameter is sufficiently small to allow it to be moved in the hole past a restriction, and an opened or expanded state in which the diameter of the tool corresponds to the desired and greater diameter to which the hole is to be enlarged below the restriction. As the tool is opened, one or more arms, hinged at their upper ends to the tool body and carrying suitable cutters at their lower ends, pivot out from the body to position the cutters for engagement of the borehole wall as the tool is thereafter operated; such operation includes rotating the tool and lowering it as it is rotated.
Underreamers are of two basic types, the so-called rock-types and drilling types. Rock-type underreamers are used where the entire length of the borehole, at least over the length thereof to be underreamed, has previously been drilled. Rock-type underreamers have large cutters which extend in the body to its center when the tool is closed; in such tools, it is not required that a circulating fluid flow axially through the tool from end-to-end. In drilling type underreamers it is required that a circulating fluid, such as air or liquid, flow from end-to-end when it is opened. Drilling type underreamers, therefore, use smaller cutters which when the tool is closed do not fully extend to the center or axis of the tool, thereby providing room in the tool for the definition of a circulating fluid duct past the retracted position of the cutters. In a drilling type underreamer the cutters are located between the exterior of the circulation duct and the exterior of the tool body when the tool is closed. Rock-type underreamers, therefore, enable a hole of given diameter to be enlarged to a greater diameter than do drilling type underreamers due to the fact that they incorporate larger cutters within the interior of the tool body than a drilling type underreamer.
A drilling type underreamer is primarily used in conjunction with a drill bit below the underreamer. The underreamer is a lower component of a string of rotary drill pipe and the drill bit is carried at the lower most end of the string. The drill bit forms the hole to be underreamed at the same time that the underreamer enlarges the hole formed by the bit. Circulating of fluid or "mud" must be provided to the drilling bit to remove cuttings and to cool the bit as a bit is operated in a borehole.
Existing rock-type underreamers enable the use of the largest possible roller cutter within the confines of the tool body and they afford maximum expanded diameter of the borehole for a given size of the tool body. Most rock-type underreamers, while they ream larger diameter holes, do not provide any communication of circulating fluid below the tool. On the other hand drilling type underreamers, while they provide fluid communication below the tool, do not provide large underreamed holes adjacent the boreholes. A U.S. Pat. No. 4,282,941 assigned to the same assignee as the present application, solves the problem of providing large cutters for large underreamed holes while at the same time providing a means to circulate fluid past the large cutters to a drill bit positioned at the lower most end of the drill string. The foregoing patent is hereby incorporated by reference.
Yet another prior art U.S. Pat. No. 4,282,942 assigned to the same assignee as the present invention and also incorporated by reference provides an underreamer apparatus that is useful in reverse circulation type drilling operations.
While both of these prior art patents have been successfully operated in the "oil patch" over the years, the large rotary cutters tend to have a limited life as they work in a borehole.
The present invention proposes to replace the state-of-the-art cutter cones which are normally steel body cones with tungsten carbide inserts inserted within interference holes formed by the cone with diamond type cutting elements. The diamond cutters are oriented on each extendable arm such that they precess as they contact a borehole wall thereby, exposing diamond cutting material to the borehole wall continuously as the underreamer is rotated in the borehole by the drill string. The use of diamond cutters in a rotary cone works especially well if the diamond material is prevented from over-heating due to prolonged contact of the diamond material against the borehole wall. The rotary cutter is freely rotatable on a bearing shaft cantilevered from the end of the extendable underreamer arm. As the underreamer body rotates, the cone precesses while it contacts the borehole wall, thus exposing new diamond continuously as the cutter cone rotates about its journal bearing.